I'm not sure if i just misunderstood its functionality, or if theres
something else going on, but i couldn't find anything about this in the
docs...

I was under the impression that the 'delprune' event
(delprune cmd="cyr_expire -E 3" at=0400)
was to prune the duplicate entry database. The last couple days, i've
come back to discover that messages in my 'old' folder are gone - with
the timestamp on the index and cache files at 4:00. Digging, this led
me to this log entry in my messages: